1. Field of the Invention
This invention pertains in general to computer security and in particular to detecting malicious software.
2. Description of the Related Art
There is a wide variety of malicious software (malware) that can attack modern computers. Malware threats include computer viruses, worms, Trojan horse programs, spyware, adware, crimeware, and phishing websites. Malware can, for example, surreptitiously capture important information such as logins, passwords, bank account identifiers, and credit card numbers. Similarly, the malware can provide hidden interfaces that allow the attacker to access and control the compromised computer.
Modern malware is often targeted and delivered to only a relative handful of computers. For example, a Trojan horse program can be designed to target computers in a particular department of a particular enterprise. Such malware is difficult for security software to detect because there are fewer instances of the same malware, and the security software might not be configured to recognize it. Moreover, even mass-distributed malware is becoming harder to detect because the malware can contain polymorphisms designed to evade detection.
In response to the increasing difficulty of detecting malware, security software is evolving toward reputation-based systems. Such systems evaluate the reputation of an entity such as a file, and use the reputation to determine whether the entity constitutes malware. Reputation systems often work well with entities that have a high prevalence, i.e., they easily identify malware detected on many endpoints. However, reputation systems work less well for entities with low prevalence because there is less confidence in the determined reputations of the entities.